1. Field of Applicable Technology
The present invention relates to structures and methods of manufacture for field emission cathodes of microtip configuration, functioning by cold-cathode electron emission, which can be formed as high-density arrays for use in such applications as matrixed flat panel display devices.
2. Prior Art Technology
When a field emission cathode is utilized as an electron source in a vacuum electronic device, it is necessary to generate an electric field strength of approximately 10.sup.6 volts/cm in order to achieve electron emission. However if such a field emission cathode is formed with a tip which has a radius of curvature of less than 10 .mu.m, i.e. is formed with a sharply pointed tip, then the electrical field that is generated as a result of applying a voltage between that field emission cathode and a corresponding electron emission electrode in a vacuum will be concentrated at the tip of the cathode. As a result, cold-cathode electron emission can be achieved with a low level of drive voltage. In the following, an element formed as a combination of such a sharply pointed cathode member and an electron extraction electrode having an extraction aperture within which the tip of the cathode member is positioned, will be referred to as a field emission cathode. The microtip cathode member itself will be referred to simply as a cathode element.
Such a field emission cathode has the following advantages, in addition to low-voltage operation:
(1) A high level of current density is achieved.
(2) Since it is not necessary to heat the cathode, the power consumption is very low.
(3) The field emission cathode can be used as a point electron source.
In the prior art, such field emission cathodes have been utilized, arranged in high element-density arrays, for example to implement a flat panel fluorescent display. This is described in the publication "Displays", P. 37, January 1987.
Prior art methods of manufacture of such field emission cathodes will be described in the following. One method is shown in FIGS. 1A and 1B. Here, an electrically conductive layer 102, an electrically insulating layer 103 and an electrically conductive layer 104 are successively deposited on an electrically insulating substrate 101, and an array of cavities 105 are formed in these superposed layers by using appropriate masks during the deposition process. Rotational evaporative deposition is then performed to deposit a suitable cathode material 106, with this rotational deposition being simultaneously executed both in a vertical direction towards the substrate and obliquely to the substrate. This results in portions 107 being formed at the upper openings of the cavities 105, and gradually closing these openings, while at the same time pyramid-shaped portions 108 of the cathode material become formed upon the electrically conductive layer 102 within each cavity 105.
Lastly, as shown in FIG. 5B, the portions 107 are removed. This method is described in the Journal of Applied Physics, Vol 39, P. 3504, 1968.
Another prior art method will be described referring to FIGS. 2A to 2F. With this method, a plurality of rectangular substrates 121 formed of an electrically insulating material are first prepared, then a film of cathode material is formed upon one face of each substrate 121. A plurality of the resultant cathode material-formed substrates 123 are then successively stacked together in a multilayer manner as shown in FIG. 2A. The resultant multilayer block is then machined on its faces to obtain a multilayer substrate block 124. Next, as shown in FIG. 2B, a metal layer 125 is formed by evaporative deposition upon a major face of this block 124, then as shown in FIG. 2C, elongated slots 126, each having a length which is almost equal to the width of the block 124, are formed in the metallic layer 125 by photo-etching. These slots extend through the layer 125, to expose respective regions of the cathode material 122. The slots 126 serve as extraction electrode apertures. The cathode material-formed substrates 123 are then mutually separated, and as shown in FIG. 2D, etching is performed on the cathode material 122 of each cathode material-formed substrates 123, to form a pattern of sharply pointed triangular portions 127. Appropriate chemical erosion is then selectively applied to the substrate 121 of each of the cathode material-formed substrates 123, to remove specific portions of the substrate 121, such that portions adjacent to each tip of a cathode material-formed substrates 123 is removed while in addition a portion of the substrate 121 adjacent to each extraction electrode aperture 126 is also removed. The cavities 128 are thereby formed in each cathode material-formed substrates 123, as shown in FIG. 2(e). The cathode material-formed substrates 123 are then once more successively stacked together in the same arrangement as that prior to being separated, and are mutually attached, to thereby form an array of field emission cathodes This method is described in Japanese Patent Laid-open No. 54-17551.
However with the first of the above prior art methods, since it is necessary to execute rotational evaporative deposition of the cathode material both in a direction vertically above the cavities within which the microtip cathode elements are formed and also in an oblique direction, the manufacturing process is difficult.
In the case of the second of the above prior art methods, in order to attain a high precision of aligning the electron extraction aperture 126 and the cathode regions 122, it is necessary to achieve a very high accuracy for the thickness of the substrate 121 and the film thickness of the cathode material thin film 122. In addition, it is necessary to position the sections of the multi-layer substrate block 124, when the block is finally re-assembled, in the respective mutual positions which the various sections had prior to being separated. However it is very difficult to achieve sufficient accuracy.